A STUDENT MICRO-PROJECTOR G. SNIDER, C . TAYLOR,
AND
SISTER VIRGINIA HEINES
Nazareth Junior College, Nazareth, Kentucky
A
MICRO-PROJECTOR is now an accepted tool in the laboratory. As a student project i t proves to be most interesting and instructive. The materials are easily obtainable in the average science department, and the set-up requires only about ten minutes. The best projectors on the market today are generally used in conjunction with a standard compound microscope, equipped with a sub-stage condenser, a 2 X to 6 X ocular and a 16, 8, or 4 mm. objective. A combination of the 4 mm. objective with a 3 X to 6 X ocular gives the best images for the ordinary specimens. For low-power subjects a 16 mm. with a 3 X to 6X ocular
is used. The image produced on the screen is eighteen to thirty-six inches. The purpose of this article is to show how easily and quickly a micro-projector may be assembled by the students for lecture work or science programs. The discussion covers the arrangement of equipment, operation, and preparation of slides for demonstration. ARRANGEMENT OF EQUIPMENT
The apparatus necessary for the construction of the micro-projector consists of a standard microscope, two bi-convex lenses, and a powerful source of light. In the drawing, (see Figure),isgivenanideaof thegeneralset-up.
The beam of light, A, may be from a carbon arc lamp, a slide projector, or a 16-mm. moving-picture projector. The latter furnished with a 750-watt lamp is by far the best to produce an intense beam of light; however, the other two will serve the purpose. Unless the arc lamp is fitted with an automatic feed, the beam has to be adjusted quite frequently. Lens B should have a
diameter of forty-four mm. with a focus of ninety-five mm., and lens C fifty mm. in diameter with a focus of one hundred thirty-four mm. The grip arm of the microscope should be tilted so that the body-tube is in a horizontal position. This arrangement must be in perfect alignment. DETAILS OF OPERATION
The source of light and the microscope are lined up about one foot apart. The bright spot is focused on the aperture of the sub-stage. The condensing lens, B, is then placed as close to the source of light as possible and lens C moved back and forth until the spot of light striking the focusing objective of the microscope reaches a maximum intensity. The lenses are then clamped to a ring stand. By far the most important part of this operation is getting the proper combination of objective and ocular. The best image produced on the screen is made by using a 16-mm. objective and a 5X ocular. This produces a bright field on the screen that measures a t least thirty-six inches in diameter a t a distance of ten feet. The tube holding the eye-piece may be removed if a smaller field is desired. The magnification is not as great, but the image is clear and sharp. Table 1 shows possible combinations and results.
1 2
3 4 5
TABLE 1 C O ~ ~ A T I OOFNOS c a m *no O e m m v ~ Oculnr Objrdioc Inogc 10 X ' 16 mm. dear, sharp. large field 6 X 6 X 5 X
5X
16 m a . 4 mm. 16 mm. 4ma.
good not as clear
best fair
After an intense field of light has been obtained on the screen, i t is time to insert the slides in the spring clips. The microscope should be steadied by means of clamps or supports to avoid any unnecessary shifting of the beam of light. A mechanical stage for the slides enables the whole field to be explored quickly and
systematically. The images can easily be brought into view by adjusting the focus of the objective. PREPARATION OF SLIDES
This projector proves invaluable to the chemistry students for the examination of crystals. It is better that the crystals be deposited on the slide from solution; otherwise, free particles will fall off as the stage of the microscope is in a vertical position. A small cover-glass sealed over the deposit will keep the samples indefinitely. The slides can be labeled and kept on file for future demonstration. Another advantage given by the micro-projector is that of viewing the specimens under polarized light. An ordinary microscope can now be equipped with polarizing attachments.* These are inexpensive and allow for the study and examination of fibers, textiles, cross-sections of plants and tissues, and optical compounds, such as tartaric acid and the sugars. A detailed account of the method of making these slides may be obtained from The Science Leaflet.' Rotatingoneof the polarizers or inserting a sheet of mica changes the colorless crystals into a profusion of beautiful tints. L M N G SPECIMENS
By using a cell slide, a drop of the biologist's culture of paramecia, amoebz, or vorticellae may be held in position by a cover slide. Imagine the enthusiasm awakened when the students see these microscopic organisms moving aaoss the screen. In order not to kill the tiuy one-celled animals by the intense heat of the beam of light, a cooling cell should be inserted between the lenses. If a projection cell is not available, a small Florence flask may be used. These living organisms may be kept alive for some time by placing a little water under the cover slide each day. To see these tiuy creatures to the best advantage, one may have to worry a while with the lenses and focus of the microscope, but the time spent is quite worth while. SUMMARY
A micro-projector as a student project may be assembled with ease. The materials necessary for this set-up are (1) a strong source of light, (2) two biconvex lenses, and (3) any standard microscope. All four parts must be as close together as possible to eliminate light leakage. A large square of cardboard or lead foil serves as a good screen. The intense beam of light passing through the two condensing lenses should be a spot of maximum brightness on the focusing objective of the microscope. The images may be thrown on a white wall or daylight projection screen. Best results are obtained in a darkened room. Slides made by the students of the science classes afford entertainment and instruction for programs.
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* The Polaroid Corporation, Boston, Mass.
1 PARSONS, G. H., "Slides for the polarizing Sci. Leaflet. 10,36-9 (Feb.4.1937).
microscope."
